In standard resistance heating of a liquid, electrical current passes through a resistive heating element that converts electrical energy into heat. The heat conducts from the hot resistive heating element to the liquid, heating the liquid. This scheme is widely used in devices such as residential and commercial water heaters, appliances, such as dishwashers, and industrial processes. In heating water, the scheme has produced problems because the surface of the resistance heating element becomes much hotter than the liquid to be heated. This higher surface temperature causes chemicals and impurities in the liquid to react, to precipitate out of the liquid, and to adhere to the hot surface of the resistance heating element, forming a lime coating on its sheathing. Over time this lime layer builds up, and acts as a thermal insulator. Thus, the now insulated resistance element gets hotter, wasting energy. As it operates at an even hotter temperature the resistance element eventually burns out. In addition, in heating of the liquid with a standard resistance heater the electrical energy dissipated in the resistor has to first heat the resistance heating element, then the resistance element's sheathing, then any lime buildup on the element's sheathing surface, and then finally the liquid. Thus, the heating of the liquid comes after some delay.
To address these problems, the lime coating on the resistance heater may be periodically removed from the appliance for deliming to prevent burn out and frequent replacement. The maintenance process of removing the mineral surface deposits takes time, adding cost and may use harsh chemicals which are damaging to the environment, costly and potentially dangerous.
Steam appliances typically found in commercial steaming applications are oven like enclosures with a door closure to seal off the oven environment to contain steam. This door is opened to insert pans or containers of food for steaming. These steam oven appliances are fed steam from a pressure boiler or steam reservoir typically outside the oven or sometimes a reservoir located in the oven. The pressure boilers or reservoirs are powered by electric resistance heaters or gas burners.
Several problems exist with these steam appliances. The boiler or reservoir is typically powered to be in a ready to steam mode for many hours a day to accommodate steaming needs, such as in a seafood department of a supermarket. A great deal of water is heated to superheated temperatures in boilers or to just below steam temperature in reservoirs to allow for full load compartment steaming whenever needed. A lot of energy is wasted by maintaining this full load steam ready condition. The door to the steam oven must be opened to insert food to be cooked. When the door is opened a lot of steam is released to the environment, more energy and water must now be used to replace this when food is inserted, the door closed and steaming is to occur. Often one pan of food is inserted to cook in a steam oven that could accommodate several or many pans of food; therefore a lot of steam is generated for cooking a load much smaller than capacity, wasting energy and water.
Since a great deal of water is heated to the maximum requirements of a steam oven, to maintain a ready condition, a lot more dissolved particles are precipitated out of the heated water to form lime scale on electric resistance elements or to metal walls of boilers. Frequent maintenance is required, using harsh chemicals, to remove lime scale and often damage is done to elements, pipes and metal walls before maintenance is done which require expensive and inconvenient service and repair. Often steamer ovens can be out of service for days in order to repair the results of lime scale.
Waste steam is typically condensed with cold water before going to drain, wasting a lot more water.
It is typical in these steam ovens to insert one pan or portion to steam and to steam for a period of time, during this period of time the door remains closed to avoid losing steam to the environment and changing the time required to complete the cook. Because of this another order or pan of food has to wait until this first cook is completed in order to be inserted and begin its cook cycle, in this way a lot of time, energy and water is wasted. Furthermore since the entire steam oven compartment has to be filled with steam in order for the cooking to begin, the cooking time is greatly enhanced.
Since the steam oven receives pans of food for steaming, and often these pans are perforated to allow steam to contact food, cooking juices and food particulate drip into the cooking compartment. It is not practical or convenient to shut down a steamer oven during hours of operation to clean between cooks and therefore all waste deposited in the steam oven compartment remain there until the end of the days cooking Subsequent pans of food are steamed in waste that accumulated during the day's operation. At the end of a day's cycle a time consuming and unpleasant cleaning is required.
Thus, better techniques for heating liquids are needed, in particular for steaming food, and these techniques are provided in this patent application.